Tokkai Hei 2002-295811 published by the Japan Patent Office in 2002 discloses a combustion device which generates high temperature gas for heating an object such as heat exchanger. This combustion device supplies fuel at plural positions in the flow direction of a lean fuel/air mixture, and generation of NOx and unburnt fuel is suppressed. The generated high temperature gas is supplied to the object to be heated which is situated downstream.